The present invention relates to photolithography, and more particularly, to the use of a pupil filter in conjunction with a binary mask to improve resolution.
Photolithography is commonly used in a manufacturing process to form patterns on a layer of photoresist. In the photolithography process, a photoresist layer is deposited over an underlying layer that is to be etched. Typically, the underlying layer is a semiconductor layer, but may be any type of substrate material. The photoresist layer is then selectively exposed to radiation through a mask. The photoresist is then developed and those portions of the photoresist that are exposed to the radiation are removed, in the case of xe2x80x9cpositivexe2x80x9d photoresist.
The mask used to pattern the wafer is placed within a photolithography exposure tool, commonly known as a xe2x80x9cstepperxe2x80x9d. In the stepper machine, the mask is placed between the radiation source and the wafer. The mask is typically formed from patterned chromium placed on a quartz substrate. The radiation passes through the quartz sections of the mask where there is no chromium substantially unattenuated. In contrast, the radiation does not pass through the chromium portions of the mask. Because radiation incident on the mask either completely passes through the quartz sections or is completely blocked by the chromium sections, this type of mask is referred to as a binary mask. After the radiation selectively passes through the mask, the pattern on the mask is transferred onto the photoresist by projecting an image of the mask onto the photoresist through a series of lenses.
As features on the mask become closer and closer together, diffraction effects begin to take effect when the size of the features on the mask are comparable to the wavelength of the light source. Diffraction blurs the image projected onto the photoresist, resulting in poor resolution.
One prior art method of preventing diffraction patterns from interfering with the desired patterning of the photoresist is to cover selected openings in the mask with a transparent layer that shifts one of the sets of exposing rays out of phase, which will null the interference pattern from diffraction. This approach is referred to as a phase shift mask (PSM). Nevertheless, use of the phase shift mask has several disadvantages. First, the design of a phase shift mask is a relatively complicated procedure that requires significant resources. Secondly, because of the nature of a phase shift mask, it is difficult to check whether or not defects are present in the phase shift mask.
Another prior art approach is to use attenuated phase shift masks (AttPSM) to enhance resolution. The AttPSM has xe2x80x9cleakyxe2x80x9d chrome features that are partially transmitting. Additionally, the light in the quartz region is phase shifted by 180 degrees. The attenuated phase shift mask operates by attenuating the zero order of light. However, one disadvantage of attenuated phase shift masks is their cost of manufacture. Additionally, it has been found that attenuated phase shift masks can create an undesirable resist loss at the side lobes of the contacts. The diffraction pattern of a square contact at the wafer, known as the Airy disk, consists of a main central intensity peak and smaller secondary peaks that are offset from the main peak. When using AttPSM, these secondary peaks are in phase with the background electromagnetic field. The intensity resulting from the constructive interaction can be sufficient to expose the resist, creating the undesired features known as side lobes.